It has long been a desirable feature of current point-and-shoot cameras to prevent accidental opening of the film-loading door while film is present in the film gate. Various mechanisms exist that mechanically sense the presence of film in the film gate and disable the film-loading door un-latching mechanism. Another type of interlock device exists for advanced photo system (APS) cameras wherein the door latch mechanism also interfaces with the film cartridge active-light-lock mechanism. This type of interlock device works by blocking the active-light-lock mechanism unless the cartridge spool is parked in the fully exposed state.
With the advent of the new APS cameras and their sophisticated electronic microprocessor controls, new customer features are being introduced. One such feature is the ability to electronically sense the fully exposed state of a cartridge that has been inserted into the camera. If, for example, a fully exposed cartridge had been inserted into the camera, the camera senses this and does not thrust the film out of the cartridge. To do so would cause double exposure of the film and resultant loss of pictures. Likewise, if a processed cartridge were inserted into the camera, the camera would likely malfunction if it attempted to thrust the film because the metering opto-sensors that are commonly used cannot detect the transitions of the film perforations that are used to register the film in the film gate for each picture.
Another APS feature enhancement is the ability of the camera to sense that a cartridge, being newly inserted into the camera, has been only partially exposed and can now be thrust in a manner that the first available unexposed frame is registered in the film gate. In each of these cases, the camera first drives the film cartridge in the rewind direction in order to optically read a cartridge bar code label and thus determine the state of the cartridge when it was first inserted into the camera. If the camera determines that the cartridge should not be used, as in the case of a fully exposed or processed cartridge, the camera must then re-park the cartridge in its original state. The camera would then provide the user some indication that the cartridge is unusable and that it should be removed from the camera.
During the camera operation cycle where the cartridge bar code label is being read, it is important that nothing interferes with the rotation of the cartridge spool. To do so would cause the bar code label to be mis-read and the exposure state of the cartridge to be incorrectly determined. The undesirable consequences could be double exposed pictures, camera malfunction, or parking the cartridge in the wrong exposure state.
Many of the APS camera features described above would present a challenge to the simple door interlock schemes currently in use. It is apparent then that a more robust and comprehensive door interlock device is required to help prevent the common mishaps that can occur and to help give the user a more enjoyable picture taking experience.
Some cameras use a large solenoid to drive the door latching mechanism or some form of motor driven gear train to provide the necessary power. While these embodiments may be robust, they tend to require substantial space in the camera or require camera power in order to remove the film cartridge at a later time. Noise is another factor that should also be considered.